Automatic telephone system



July 19, 1932. R, L T K L 1,868,296

AUTOMATIC TELEPHONE SYSTEM Filed Sept. 25, 1931 s Sheets-Sheet 1 II TH. nomnr STEP INVENTOR R. L. 5 TOKELY BYWWM A 7'TORNEY July 19, 1932. R. 1.. STOKELY AUTOMATIC TELEPHONE SYSTEM Filed Sept. 25, 1951 s Sheets-Sheet 2 A TTORNEV INVENTOR R. L. STU/(ELY B) W km July 19, 1932. R. L.-STOKELY AUTOMATIC TELEPHONE SYSTEM 3 Sheets-Sheet 3 Filed Sept. 25, 1931 mvg/v TOR R. L. S TOK E L) A TTORNE) Patented July 19, 1932 TJNETED STATES PATENT OFFICE RAY jfsrorintr, or FLORAL PARK, NEW YORK, ASSIGNOR 'ro BE 'I. TELEPHONE LAB- vonn'ronrns, INCORPORATED, or new YORK, N. Y., A CORPORATION 'OFNEW'YORK AUTOMATIC TELEPHONE SYS TEM Application filed September25, 1931. Serial No. 565,066.

Thisinvention relates to telephone systems and particularly to systems employing auto matic switches for establishing connections between subscribers lines.

5.5 The objects are to safeguard the system against false connections, to enable the economical use of equipment without impairing the quality of the service, and to otherwise improve the operation of systems of this character.

It has been the practice heretofore to effect the release ofa train of automatic switches by means controlled partly by the calling subscriber and partly by the called subscriber.

WVhere switches of the step-by-step type are employed, the holding ground potential for maintaining the connection is usually applied atthe connector switch. To permit the release of all switches except the. connector when the calling party replaces his receiver,

it is necessary to temporarily remove the holding condition from the connector switch; and to prevent some other selector from seizing this connector, while it is thereafter-held by the called subscriber, it is necessary that the holding or busy condition be reapplied at the end of a short interval. Although the interval during which the busy condition is removed .from the connector is short,there is. a possibility that some selector switch searching for an idle connector will seizethis unguarded connector and thus bring about .a false connection. WVhen the connection ineludes one or more impulse repeaters, the outgoing repeater in the originating office furnishes the holding ground for the switches located in that office. In this case it is also:

necessary that the holding or busy ground on the sleeve conductor of the outgoing re peater be temporarily disconnected, when the calling subscriber replaces the receiver, to permit the release of the switches in the originating oifice; and since faulty operation would result if the. outgoing repeater were again seized on another connection before the selee-tor switches in thedistant office have reached their normalposition, it is neces-.

before these switches have restored to normal.

According to the present invention, means are provided to prevent the seizure of an outgoing repeater ora connector during the short interval in which their respective sleeve 5 I conductors are freed of the holding or busy potential, when the receiver is replaced upon the switchhook at the calling station, to permit the release of the; preceding switches.

Thesemeans include a relay which is connected to thesleeve conductor atthe time that the busy potential is removed. If there is no attempt to ,seize the repeater or connector during these unguarded intervals the aforementioned relays remain inert and the busy potentials are not reconnected to the sleeve conductors until the 'usual intervals have elapsed; if, however, a hunting selector attempts to seize the repeater or the connector during the unguarded interval, the associated I relay is immediately operated to reapply the guarding potential :and thus prevent the threatened seizure of therepeater or connector.

The invention will be better understood by considering the operation of a telephone system in which its features have been lIlCO1-, porated, such a system being represented schematically in the drawings which form a part of the following description. It is and 3' represent schematically a multi-oflice telephone system in which automatic switches are employed for establishing connections between the SllbSCIlbGIS; I

Fig. 1 shows a calling subscribersstation A, -a line-finder switch LF, a firstselector switch S1, and a second selector switch S2. Fig. .2 shows :an outgoing repeater -OR, a

trunk T, an incomingrepeater IR and 1a 'selectorS3, all three of these circuit units. being permanently associated withv each othe F ig. 3 shows a connector circuit-C, anda called subscribers station'B.

The subscribers stations A and B are provided with the usual subscribers telephone sets and with a dial for controlling the establishment of desired connections.- The linefinder LF is of the well known Strowger type and'may be similar to that disclosed in Patent 1,711,682 issued May 7, 1929 to H.

Hovland. The brushes 2, 3 and & correspond toione ofthe two sets of brushes shownin the Hovland patent; .The selector switches '81, S2 andS3 and the" connector switch C are also of the well known Strowgertype and reference may be had to pages 53 to 67 inclusive, of the second'edition of Automatic coming repeater IR; and the incoming re-' 7 Telephony by Smith and Campbell for a detailed description of the operation 01" these switches. Only those portions ofthe circuits of the line-finder LF, selectors S1 and';S3 are shown as are required for a clear and complete description ofthis invention, the omitted portions of these circuits beingrepresented by broken. lines. The selector S2, the outgoing repeater OR, the incoming repeater IR and the connector G are shown in detail." U

- The outgoing repeater OR is arranged to repeatthe dial impulses from acalling station over the two wire trunk T to the inpeater IR isarranged to. repeat the impulses rece1ved over trunk T to the succeedmg selector and connector switches used in estab-- lishing the desired connection. The repeater OR is further arranged in accordance with this invention to prevent its seizure on a second connection during the interval in which the holding ground isremovedfrom the test conductor to release the preceding switches. The connector switch G is'also arranged in accordance with'this invention to'prevent its seizure on a second'connection during the interval in which the holding ground is removed from the test conductor to release the preceding switches. Theinvention will be better understood from a description of the operation of these circuits in establishing a connection between the stations A and B." i I I Assuming thatthe subscriber at station A removes-the receiver from the receiver hook 'to' originate the call, the line-finder LF hunts for and extends the line 1 through its multiple brushes 2, 3 and l to thefirst selector S1. WVhen the calling subscriber dials the first digit. of the calling subscribcrs number, the brushes 5, 6 and 7 of selector switch S1 are advanced 'to a corresponding group of terminals. The selector S1 then automatically hunts'for and seizes an. idle set of terminals; in the selected group. As-

' suming that theselected'set of terminals is connected tothe selector S2, the line relay 11 is energized ina circuit which maybe tracedv from battery through the lower wind-v ing of relay 11, outer lower back contact of relay 18, conductor 9, brushes 6 and 3 of the L selector S1 and line-finder LR'over one side of the calling line 1, through the subscribers setat station A, backover the other side "of line 1, through brushes 2 and 5 of the line finder LF and selector S1, over conductor 8, through'the outer upper backcontact of relay 18, upper winding of relay 11, and through the. normally closed contacts of the 11th rotary-step. springs to ground. 'Relay 11' closes an obvious circuit for operating relay 12. Relay 12 connects ground through its lower front contact, over test conductor 10, through brush 7, of selector S1 to hold the line-finder and first selector switches in their operated positions.

subscribers number, relay 11 is alternately released and reoperated in accordance with Each succeeding release and reoperation of relay 11 causes a corresponding operation and release of magnet 14 to' advance the brushes 25, 26 and 27 to the levelcorrespond' ing to the digit dialed. Being slow in releasing, relay 13 remains operated until all of the impulses created by thedialing ofthe second digit have: been received. Relay 12 does not release duringthe receipt of dial impulses since this relay is also slow. 1n releasing.

When the'shaft of switch S2 moves'out of its normal position, the verticalofii-nor-i mal springs VON are actuated thereby closinga. circuit from battery through the wlnding of relay 16, the upper contacts of tact of relay 13, over conductor 10 to ground at-relay 12. Relay 16 operates in this circuit and closes a locking. circuit, independent of relay 13, from conductor22 through theback contact of the rotary stepping magnet 17, over conductor 24, through the lower front contact of relay 16, over conductor 23,

to ground at theinner upper back contact of relay '18. 1 When relay 13 releases after all magnet 17 operates in circuit which may be traced from battery through'the winding of magnet 17, upper front contact of relay the VON springs, conductor 22, front conol the'imp'ulses corresponding to the second digit have been received, the rotary stepping 16, back contact of relay 13, overconductor 10 to ground'at'relay 12., The operation of magnet 17 rotates brushes 25,26 and27 into 7 80. I When the calling sub- 1 scrlber dials the second digit'of: the calledcontact with the first set offterminals in the selected level. Relay 16 releases when magnet 17 operates, and the release of relay 16.

back contact of magnet 17, conductor 24-,

winding of relay 18, upper contacts of the 11th rotary-step springs, over conductor 10 to ground at relay 12. If the first setof terminals is busy, the winding of relay- 18 is short-circuited by the busy ground on the test terminal, this short-circuit being traced through brush 27 of selector S2 and the inner lower back contact of relay 18. Relay 16 reoperates over this short-circuit to the busy ground, and again closes the circuit for operating the rotary stepping magnet 17 The operation of magnet 17 opens the circuit for operating relay 16, and the release of relay 16 again causes the release of magnet 17. Inthis manner relay 16 and magnet 17 are alternately operated and released to step the brushes from one set of terminals to the next until an 'idleset' of terminals is reached, at which time relay 18 is operated in the, circuit hereinbefore traced. Relay 16 is marginal and does not operate in series with relay 18. With relay 18 operated, a busy ground is connected from conductor 10, through the inner lower front contact of relay 18, and through brush 27 to the test terminal of the selected set so as to prevent seizure of a corresponding set of terminals by any other hunting selector.

If all of the trunks in the selected level are busy, the eleventh operation of magnet 17 advances thebrushes beyond the tenth set of terminals to cause the operation of the 1 11th rotary-step springs and thereby connect the secondarywinding of the busytone transformer 15, through the front con tact of the 11th rotary-step springs, to the upper winding of relay 11, and thus transmit a busy tone to the calling subscriber.

v Assuming the selected set of terminals to be connected to the outgoing repeater OR, the

operation of relay 18 of the selector S2 extends the connection from the callingline over conductors'8 and 9, through brushes 25 and 26, and over conductors 28 and 29 to the repeater OR, operating the line relay 31.

The circuit for relay 31 may be traced from battery through its upper Wmdin g, the inner upper back contact of relay 38, conductor 29,

brush 26 of selector S2, the outer lower front contact of relay 18, conductor 9, through the brushes of selector S1 and line-finder LF and over the callin line, back through conductor 8, the outer upper front contact of relay 18, brush 25 of selector S2, conductor 28, the inner lower back contact of relay 38, and'through the lower winding of relay 31 to ground. Relay 31 closes an obvious air-- cuit for operating the holding relay 32. Re- 7 lay 31 also closes a circuit for operating rethrough the winding offrelay34, the inner 'lay34, this circuit beingtraced from battery upperback contact of relay toground at the outer upper front'contact oflrelay 31.

lower front contact, over conductor '30, to the sleeve terminal and brush 27 0f selector .Relay 32 connects ground through its "outer,

S2 to hold the preceding switches operated J until the connection is released by the calling subscriber. Relay 32 also closes a bridge across the conductors of the outgoing trunk T, thereby operating the line relay 51 of the incoming repeater IR. This bridge may be traced from the upper conductor of trunk T, over conductor 49, through the .inner front contact of relay 31, conductor .47, upper winding of-relay 37 conductor 46, the outer upper front contact ofre'lay. 32, over conductor 48, tothe lower conductor of trunk T. Relay 37 is polarized and .does not operate by the current in this circuit at this time.

With relay 34operated, an obvious circuit is closed for operating relay 35 and ground is disconnected from the conductor which leads-to the all-trunk-busy register circuit 39.

Relay 35 opens the operating circuit for rei lay 34, but relay 34 is held operated through its upper outer front contact to ground at the innerlower front contact of relay 32.

When the calling subscriber dials the third digit of the called subscribers numbenrelay 31is alternately released and reoperated in accordance with the impulses thus created. The first release of relay- 31 in addition to opening the aforementioned bridge across.

the conductors of trunk T closes a circuit for operating the slow-to-release relay 33. This circuit may be traced frombattery through the winding of relay 33,"upper innerfront contact of relay 32, to ground at the back contact of relay 31. Relay 33 disconnects the upper one of the talking condensers 10 from trunk T and closes a short-circuit around the upper winding of relay 37 ;'this shortcircuit may be traced from conductor 46, through theupper front contactof re y 5 i .33 and'the outer upper back contact ofrelay 38, to'co'nductor47. l Vhen'relay 31 reoperates atfthe end of the first impulse, the

bridge is again closed across trunk T. Each release and "reoperationof relay 31 is thus effective to open and close thebridge across theoutgoing trunk], to repeat the incoming impulses to the'line relay 51 of the incoming repeater IR. Being slow in releasing,'relay 32 does not release during the receipt'of dial impulses. Relay 33 is also slow to release and remains operated during thelreceipt of each train of impulses. When relay 33'releases, the short-circuit across 7 Winding of relay 37 is opened. V e "-The' operation of relay51 of theincoming V the upper use 1 repeater IR, due to the closureofthe bridge 7 acrossthe conductors. of trunk T, connects fcharge condenser 61 and cause the operation 7 the winding of relay 56 to one side of condenser 61. Since this condenser is normally short-circuited bythe back contact of relay- 51, the operation of relay 51 causes acurrent to flow through thewinding of relay 56 to of relay 56. The charging current is temporary' and relay 56 remains operatedfor 1 onlya short interval of time. The operation of relay 51 also closes acircuit from battery through the winding of relay 52 and the winding ofrelay53, to ground at'the outer front contact of,relay 51. Relay 52 operates in this circuit but relay58 is slow to operate and, before it issufhciently energizedto actuate its contacts, its winding becomes shortcircuited by the contacts ofrelay 52. Thls short-circuit is traced from ground at the upper front contact of relay 52, over conductor 67, the back "contact of relay 54, and

through the inner lower front contact of re-' lay 52 to the conductor which connects'the windings of relays-52 and 53. Thisconnection not only short-circuits the winding of relay 53 but'also constitutes a temporary locking circuit for relay 52. With relay 52 operated, a circuit is closed'from battery through the upper winding ofrelay '55, over conductor 67 to ground at the upper front contact of relay 52; being polarized relay 55 is not operated since the current in this circuit is not in the operating direction. The ground thus connected to conductor 67 is also p extended through the backcontact of relay 1 V at the outer front contact of'relay 51. lVith' 55 to the winding of relay 54 causing the operation of relay 54. Relay 54 opens the aforementioned locking path for relay 52 and short-circuit for relay 53, so that relay :53 is now operated and relay 52 is held in series with the winding of relay 53Qto the ground relay 52 operated, the ground at its outer lower back contact is disconnected from the conductor which leads to the all-trunks-busy register circuit 59. With relay 53-operated,

' the talking condensers 50 are connected to conductors 63 and 64, and-the lower contacts I Z r of relay 5?) are ineffective to short-circuit the winding of coil 60. NVith relay 54 operated,

. a circuit is closed from battery-through the erated through its inner front contact and,

the back contact of' relay: 56to the ground on conductor 67'. The aforementioned operation of relay 54 is'also effective to close a bridge across conductors 63' and 64 which leadzto the selector S3, thus causing the iopconductor 63, through theouter front con- 7 'tact of relayr54, winding of polarized relay 57, winding of retard coil 60, outer lower front contact of relay 52, to conductor 64.

Relay 57 is not operated sincefthe current in this circuit is not'in the operating direction. Inresponse to the impulses created by the her and repeated by the outgoing repeater of relay 51 opens the circuit, through the ing the release of relay 53 but relay 52 remains operated, during the receipt of impulses due to its 'beingrslow to release, Relay 53 does not reoperate until all of the impulses havebeenreceived due to. its being slow to operate. Relay 53 is thereby effective to shortcircuit'the windings of polarized relay 57 and: retard coil 60 during the transmission-of each set of, impulses, and is also effective to disconnect the talking condensers 50 from the outgoing impulse circuitfl Each release and reoperation of relay51 disconnects and again connects the winding of relay 56' to corn denser 61, so that relayg56 is temporarily operated each time relay 51 operates. The operation'of relay 56, in response to the receipt of the first impulse, opens the locking circuit for relay 54 thereby causing its release; /Vhenrelay 54releases, the bridge across con- 'ductors 63 and 64 is opened to cause the release of the line relay of selector S3. The release of relay 54 also opens the circuit through the lower windingof relay 55; but relay 55 is slow in releasing due .to current continuing to ilow'througnthis winding to charge con-v denser 62. When relay 55 releases, the wind ing of relay 54 is again connected to ground on conductor 67 thereby causing thereoperation of relay 54. The release of relay 56 after receiptofthe first impulse againcloses the locking circuit for relay 54s With'relay 54 'reoperated, the bridge is again closed across conductors'63 and 64; and the circuit through the lower winding of relay,55 is again closed to cause: the operation of relay 55'. Relays 51, '53, 54, 55 and'56 repeat thiscycle of operationsin response to each impulse received over trunk T so as totransmit' a correspond- ;ing impulse over conductors63 and 64 to selector $3. Succeeding trains of impulses created by the dialing of the remainlng digits of the called number mean like manner received over trunk Tand repeated overconduc-v tors 63 and 64 to control the operation of the succeeding switches required for the establishment of the desired connection. The resist-r ances and condenseres associated with relay 55 control the releasing time of relay 55 so i dialinglof the third digit of the called nurnwindings of relays 52 and 53 thereby caus 39' and the other succeeding, switches; Atthe end of each series ofimpulses relay 53 reoperates to connectconductors 63 and 64; through condensers 50 to the conductors of-tr unk T. In response to the impulses created bythe dialing of the third digit, and repeated by,- the out- 01 1; 1

idle set of terminals in the selected group.

Assuming that the terminals thus selected-are connectedto the connector 0, the line relay 71 operates in a circuitwhich may be traced from battery through the upper winding .of relay 71, the lower back contactvof relay 7 8, over conductor 69, through brush 66 of selector 553, over conductor 64, through the outer lower front contact of relay 52, winding'of retard coil 60, winding of polarized relay 57 outer front contact of relay 54, overconductor 63, and through brush of selector S8, conductor 68, upper back contact of relay 78, and through the lower winding of relay 71,- to ground. Relay 71 closesan obvious circuit for operating the slow-to-release relay 72. Relay 72 connects ground through its outer lower front contact over sieeve conductor 70,

and through the sleeve brush of selector S3 to hold this switch in its operated position;

Theimpulses created by'the dialing of the last two digits of the called subscribers number, repeated by the outgoing repeater OR and the incoming repeater IR, now cause the selective operation'of the connector switch G. The release of relay 471, in response to the first impulse received from repeater IR closes a circuit from ground through the back contact of relay 71, they 7 upper front contact of relay 7 2, the normally closed contacts of the vertical off-normal springs VON, the winding of relay 7 3 and the winding of the verticalstepping mag net 7 4, to battery. The magnet 74 andrclay 73 are operated in this circuit, the operation of the magnet stepping up the shaft of'this switch one'step, so that thebrushes 95, 96

and 97 are opposite the terminals in the first level. The vertical off-normal springs VON are actuated when-the shaft is moved'out of its normal position and the clrcuit through the winding of magnet 74 and relay 73 is now traced through the front contact f relay 73 and the lower front contact of the vertical off-normal springs, through the front contact of relay 72, to ground at the back contact of relay 71. When relay 71 reoperates at the end of the first impulse, magnet 74 releases. Each succeeding release and rcoperation of relay 71 causes the operation and release of magnet 74, thus advancing the shaft and brushes of connector "of terminals in the selectedlevel.

switch C to the level correspondingto 'the' fourth digit of the called subscribers c-num- 7 her. Relay 73 isslow' in releasing and re-V mains operated. duringreceipti of this train of impulses. Relay 72 is also slow in'releasing and-remains operated 'duringthe receipt ofthis andlthe succeeding train of impulses. o v v a 1 r a When relay 71 releases in response-to'ther firstimpulse of thelast series, acircuit. is

closedzfrom ground at the back contactlof relay 71, through; thecupper front contact of relay 72, the lower front contact of the VON springs, back contact of relay 73, 1

magnet 76 are operated inthis circuit, the

operation ofmagnet' 76 rotating the brushes 95, 96 and 97 into contact with the first set connects conductor 82 through-its left-hand front contactto the back contact of relay 73 thus'making the circuit throughfthe winding of-relay 75 and the winding-ofmagnet 76 independent of relay 80. -Each succeeding release and reoperation of relay 71 is effective to cause'the reope-ration and release Relay 75 J of magnet7 l, thus rotating thebrushes 95, i

.96 and 97 into contact with a set ofterminals corresponding torthe-last digit of the called [subscribers number. Relay 75 is slow" to release andremains operated duringthe re ceipt of the entire train of'impulses; With sleeve brush 97 in contact with thelsleeve terminal ofthe called line, a circuit is closed from batterythrough the winding of'busy test relay 80, over conductor 84, through the inner lower back contact of relay 77 the right front contact'of relay 75, over conductor 88, through brush 97, to the test terminal 98. If the. called line isidlerelay 80 does not operate; but'if the called'line is busy, the test terminal 98 is connected to groundland relay 80 operates. Withrelay 80 operated,'the secondary winding of the busy transformer 81 is connected through the inner rightehand front contact of relay 80, over conductor 92 to condenser 90, thereby transmittinga busytone to the calling subscriber. V

Assuming that the called line is idle, the

release'of relay 75 closes a circuit from the lower front contact of relay 72.

77 is thus fully energized to actuate its re.

mg, over conductor sate grmind at the inner Relay 'maining contacts and thus extend the talking conductors of line 99through brushes 95 and96, overconductors' 93 and 94 to the ringing relay 79. Relay 77 connects a holding and, test-busy ground through its next-tothe-outer-lower front contact, over conductor 88 and through brush 97 tofterminal 98, to

hold the aforementioned cut-off relay and prevent seizure 'of the line 99 by any other connector having access thereto. Ringing current "is-now connected to the :line 99 to signal the called subscriber. The ringing cir 'cuit maybe traced from ground through the source of ringing current 86, the interrupter 87, the inner lower front contact of relay 7 7 the lower winding and lower backcontact of relay 79, the lowermost front contact of relay 77, over conductor 94', through brush 96, over,

from the' receiver-hook, the current flowing 1 through the lower 'winding of relay 79 is increased sufliciently to cause the operation,

of this slow-to operate relay.'- Relay 79 locks through its upper winding and inner upper front contacts to the ground on conduct-or 83. With the relay 79 operated, the talking connection is extended from conductors 68 and 69, through condensers 90, over conductors 91-and 92,'through the outer front contacts of relay 79,. the outer front contacts of relay 77, over conductors 93 and 94, through brushes 95 and'96 to the called line 99. Talk ing'battery is supplied to the called sub- 7 s'cribers station through the windings of relay 78 in the usual manner. Relays 78 ope erates thereby reversing the: connections between conductors 68 and69'and the windings of relay 71 thus causingthe reversal of current over conductors 68 and 69 to operate the polarized supervisory relay 57 in the repeater IR. Relay 57 closes an obvious circuit for operating relay'58; Relay 58*reverses.

the connections between the conductors of trunk T and the windings of relay 51 thus reversing {the flow of current over the trunk T to cause the operation of polarized supervi'sory relay 37 in the outgoing repeater OR.

Relay 37 in turn closes a circuit for operating relay 38; and relay 38 reverses the connec tions from conductors 28 and 29 with respect to the windings of relay 31. The current is thus reversed in the circuit over conductors 28 and 29, through the talking conductors of selectors S2 and S1 and linefinder LF, and over the calling line in the usual manner. g Y

,xVVhen. the receiver is replaced on the receiver-hookat the calling station, the line,

relay 31 of the outgoing repeater OR releases,

thereby causing the release of relay 32.. Relay31 also opens the bridgeacross the conductors of trunk T to release line relay 51' of the incoming repeater IR. Relay 32 disn-ects this conductor through the lower front contact of relay 35 to the'winding of relay 7 36. Relay'32 also opens the circuit through connects the test conductor 30 from the ground at its lower front contact and conthe winding of the slow-to-release relay 34:.

When relay 34 releases, it closes a circuit from ground through its inner back contact,

the inner upper front contact of relay. 35, w

and throughthe winding of relay 32 to battery; Relay 32 reoperates in this'circuit and thus-reconnects the ground at its outer lower front contact to test conductor 30 as a guardingpotentia-l. With relay '32 operated and K relay 31 released, relay 33 reoperates. The I aforementionedrelease of relay 34 is also effective to cause the release of the slow-torelea'se relay 35; and when relay 35 releases,

ground'isalso connected from the lower front contact of relay 33 through the outer back contact of relay 35 to test conductor 30. The 7 release of relay 35 also opens the circuit which caused the 'reoperation of relay 32, so that relay 32'again releases, thereby causing the 8 release of relay '33 With relays 32 and 33 both released, the test conductor 30 is no longer connected to ground and the repeater maynowbe seized for use on a succeeding call. The interval during which the test eonductor- 30 is freed of'the holding ground is sufficient to insure the release of relay 18 of selector S2 and the corresponding relay in each of the preceding selector switches to cause the return of these switches and the line-finder switch to normal in the usual man- 7 1 ner. The circuit for operating the release magnet 19 of selector S2 maybe traced from battery through its winding, lower contacts a of the VON springs, and upper back contacts of relays 12, 11 and 18 to ground. This circuit is opened by the VON springs when the shaft and brushes of selector S2 reach normal. 7 1

- In case some selector havlng access .to the operates; the circuit for operating relay 36 repeater should test conductor 30 while. relay 32 is temporarily released, relay 36 may be traced'from, battery through its winding, the lower front contact of; relay 35', the back contact of relay 32, over conductor 30,

.through the test terminal and test brush of the selector similar toselector S2 which is hunting for an idle outgoing repeater, through the lower inner back contact and winding of a relay corresponding to relay 18, and through the back contacts of the 11th 'rotary-step springs of the hunting selector to ground, at the lower front contact of a relay corresponding to relay 12. Relay 36 isa fast operating relay and'ground is connected through its front contact and the back contact of relay 82 to act as a Ltemporary'busy potential and thus prevent the operation of the relay in the hunting selector which corresponds to relay 18. Relay 36 locks and remains operated until relay, 85 releases.

:The seizure of the repeater OR. by another selector prior to the reoperation of relay 32 is thus prevented to insure that the bridge across trunk T be held open by relays 31 and 32 for an interval sufficient to cause the release of the succeeding selectors and also the release of the connector-C- if'the receiver has also been replaced on the receiver-hook at station B. I r

The aforementioned opening of the bridge across trunk T causes the release of relay 51 of the incoming repeater IR. lVith relay 51 normal, relays 52 and 53 release. With relay 52 released, r lays 54 and 55 release. The bridge across conductors 63 and G l-is opened when relay 52 releases, thereby causing the release of the line relay 71 of the connector C. The release of relay 71 closes the circuit for operating relay 75 and opens the circuit for relay 72. If the receiver at the called station has not been replaced on the receiverhook relay 78 of the connector remains energized to. hold relays 7 7 and 7 9 and thus to prevent the return of the connector switch to normal. To release the select-or S3, the release of relay 72 disconnects the holdingground from test conductor and thus releases the relay (not shown) of selector S3 which corresponds to relay 18 of the selector S2. The selector S3 is thus restored to normal in the usual manner. With relay 72- released, the test conductor 70 .is connected through its lower back contact. over conductor 84 to the.

winding of relay 80 and the circuit through the winding of relay 75is opened. Therelease of relay 7 5 connects ground at the nextto-the-outer lower front contact of rela'y 77, through the outer right back contact of relay 75. lower winding of relay 77, and through the right back contact of relay to conductor'70 to prevent the seizure of connector 0; thisground connection is also extended from conductor 70 through the. lower back contact of relay 7 2 to the-winding of relay 80 thereby causing the operation of relay 80. Relay 8O locks through its left frontcontact to ground at the middle lower front contact of relay 78 and thus extends this ground to test conductor 70. In this manner the resistance of the lower winding of relay 77 is eliminated from the busy potential connection to render the protection against seizure more effective.

In case a hunting selector attempts to seize the connector Cduring the interval of time between the release of relay 72 and the release of relay 75, relay 80 operates in a circuit which may be traced frombattery through its winding, conductor 84, lower back conreaches its normal position.

middle 'lowerfront contact of relay/78, through. the left front contact of; relay 80.

over conductor Set to test conductor 7 O to prevent the operation of the test relay of the hunting selector and thus cause this selector 7 to advance beyond the terminals of. the connector C. Relay 80 is now held operated under the control of relay 7 8 until the called subscriber replaces the receiver on the switchhook, at which time relays 7 8, 80.7? and 79 release. The release of relay 77 closes the circuit for operating the release magnet 89. he shaft and brushes of connector. are then returned to normal in the, usual manner, the circuit for magnet 89 being opened at the contacts of the VON springs when the shaft .Wh'at is claimed is: 1. In combination, a trunk, meansfor extending connections to said trunk, means for 'establishinga busy condition to guard the trunk against seizure, means for removing 25 said condition, and means for reestablishin i' said condition in response to an attempted seizure of the trunk by said connective means. 2. In combination, a trunk terminating in a selective switch, means for extending con-- nections to saidtrunk, means for establishing a busy condition to guard the trunk a gainst seizure, means for removing said condition, and means for reestablishing said con dition inresponse'to an attempted seizure of the trunk by said connective means. 3. In combination, a selective switch, automatic switches having accessto said selective switch for extending connectionsthereto, means associated 1with said selective switch for establishing a condition on the extended connection to hold one ofsaid automatic switches in its operated position; means for removing said holding condition to permit the release of the operated automatic switch, and. means' for reestablishing said condition in response to an attempted Seizure of said selector switch by one of said auto-' matic switches.

4;. In a telephone system, subscribers lines. a link for use in connectinga' calling and a called line, means'fc r extending a calling one of said lines to said link, means for establishin g a'busy condition to guard the link against seizure means for removing said condition, and-means for reestablishing said condition in response to an attempted seizure of the link by said connective means. a 5. In a telephone system, subscribers lines,

called line, prececiiig links having access to 7 link. one of said preceding links connectcal ing one ofsaid lines to said link,

preceding links, means for removing said busyiconditionto cause the release ofsaid link by said one of the preceding links, and .means for reestablishing the'busy condition in response to an attempted seizure of said linli byanother one of said preceding links. j 6.1n a telephone system, a link, switches each having access to saidlink, means respon- *switchesduring said interval toconnect a guarding potential to said conductor to'preventsaid attempted seizure from being successi'ul.

'7. The combination a telephone system of a connector switch, selector switches for extending connect-ions to said connectorswitch, means associated withsaidconnector for establishing a holding condition on the connection extending back to one of said selector "switches, means for removing said holding condition for'aninterval of time to ermit the release of the precedingselector switch, means effective after the expiration of said interval for reestablishing said condition to guard the connectoragainst seizure,

and .means effective during said interval to reestahlishsaid condition should another one of said selector switches attempt to seize said connector. w 8. The combination in a telephonesystem of a'calling line and a called line,a selective switch and other switches having access to said selective switch, means for'opcrating said switches to establish a connection between the calling line and the called line, means in said selective switch for setting up a condition in the established connection to hold said other switches in operated position, means controlled by the calling subscriber for removing said holding condition to permit the release of said other'switches, means effective to restore said condition in response to an attempt by one of said other switches to seize said selective switch, and means controlled by thecalled subscriber for releasing said selective switch. 7

9. l-he combination in a telephone system ofa callingline and a called line, selector.

, c responsive to the seizure 1 of said link. for establish ng a bnsyconcition to guard said link seizure byanother of said switches and a connector switch for establishing a connection betweensaid lines, means in the connector switch for applying a given potentialto. the connection tohold said slector switches operated, meansvcontrolled by the calling subscriber for momentarily removing said potential to permit the release of said selector switches, means in said connector switch eiiective in response to 1 an attempted seizure ofsaid connector switch by one of said selectors to restore said poten-' tial to guard said connector switch from seizure, and means controlled, by the called subscriber for releasing said connector switch. r r

10. In combination, an automatic switch, a connective circuit extending to said switch,

other switches having access to said'connec tive circuit, means responsive to the operation of said automatic switch for establishing a holding condition on one of the conductors of said connective circuit, means for removing said holding condition to permit the release of one of said other switches, a relay, means for connecting said relay to said conductor when said condition is'removed therefrom, means for' operating' said relay responsive to an" attempt by one of said {other switches to seize said automatic switch and a circuit completed by said relay for restoring said conditiontoguardthe automatic switch against seizure.

11. In combination, an automatic switch,

a circuit including a control conductor extending to said switch, other switches for tial on said control conductor to hold preced- ;ingswitches, means for removing said ground potential for an'interval of time to permit the release of preceding switches without restoring said automatic switch, a relay associated with said automatic switch,

means for connecting said'relay to said hold ing conductor during said interval of time, means for operating said relay ifa preceding switch attempts to seize said automatic switch during said interval, means controlled by said relay for restoring said ground potential to the control conductor to prevent the seizure of said automatic'switch, and

seizing said circuit to extend connections to [said automatic switch, means in saidautomat- .ic switch for establishing a ground potenmeans for restoringsaid ground potential to the control conductor after the expiration of said interval of time. a i y, 12. In a telephone system, subscribers line s,a repeater 'for useinflextending a connection from a calling to a called line, switches having access to said repeater, switches for operation in response to impulses transmittedthroughsaid repeater from a calling line, means responsive to the seizure of said repeater byone of said preceding switches for connecting a guarding potential to one of the conductors of theflrepeater,

means for removingsaid guarding potential, and means responsive to an attempted seizure of'said repeater by another one of said preceding switches for reconnecting said guarding potential to prevent said attempted seizure from being successfuL 18. In a telephone system, subscribers lines, a repeater for use in extending a connection from a calling to a called line, swltches havlng access tosa1d repeater,. swltches for operatlon in response to 1m-' pulses transmitted through said repeater from a calling line, means responsive to the seizure of said repeater by'one of said preceding switches for connecting a guarding potential to one of the conductors of the re- P peater, means responsive to the release of the connection by the calling subscriber for causing the release ofone of said succeeding switches and for temporarily disconnecting said guarding potential to release said one of said preceding switches; and means effective in response to an attempted seizure of said repeater while said guarding potential is temporarily disconnected for reconnecting a guarding potential to said conductor to pre-.

vent said attempted seizure from being suc cessful. V

In witness whereof, I hereunto subscribe my name this 22 day of September, 1931.

' RAY L STOKELY. 

